This invention relates to cylindrical waveguides. More specifically, this invention is a waveguide coupler used to couple a portion of the energy propagated through one waveguide into another waveguide.
It is frequently necessary to sample the signal being transmitted through a waveguide, for example, to measure standing wave ratios or output power from a transmitter. Prior art waveguide couplers coupled radio frequency energy in one waveguide into an adjacent waveguide by physically placing two rectangular waveguides together so that the walls of the two waveguides in contact form a common wall through which slots or apertures are cut at predetermined intervals to permit electromagnetic energy in one waveguide to radiate into the other waveguide. The slots or apertures cut in the common wall of the two waveguides generally have predetermined geometries that permit energy transfer from a first waveguide into a second waveguide such that directional wave propagation will occur in the second waveguide. These spaced apertures or openings require close machining tolerances and precise spacing to accomplish an efficient energy transfer from one waveguide to the next.
Frequency dependency constitutes another problem with prior art couplers. Apertures of any given size and spacing permit more coupling at higher frequencies than they do at lower frequencies. When employed in broadband applications, a coupler, using spaced apertures, will couple different frequencies, at different levels.
Spaced apertures might also exhibit two-directional signal propagation in the coupled waveguide, when the slots or apertures are not spaced 1/4 of the wavelength of the coupled signal (i.e. signals in the coupled waveguide might propagate in both directions). This bidirectional coupling occurs when wavefronts in the coupled waveguide do not properly add in the desired direction and the wavefronts in the coupled waveguide do not properly cancel in the opposite direction, all because of inexact slot spacing.
When one waveguide is used across a relatively wide range of frequencies, the level of coupling might change substantially from one end of the frequency range to another when using only a single set of apertures for the coupler. When using spaced aperture couplers with a waveguide that carries signals across a relatively wide frequency range, it is frequently necessary to cascade many pairs of coupling apertures, each optimized for small segments of the frequency range. An alternative is to cascade separate frequency couplers, with each coupler being optimized for a frequency band. Of course a disadvantage of having to cascade several spaced aperture couplers in a waveguide system is the added cost, weight, and complexity of the transmission line. A waveguide coupler which is inherently less frequency dependent and easier to fabricate would be an improvement over the prior art.